Voltage regulators receive an unregulated input voltage signal and output a substantially constant voltage signal. In other words, the voltage regulator regulates the input signal so that it can be used by other devices (e.g., cell phones, music players, voltage sensitive devices, computers, etc.). The input voltage signal and the output voltage signal may be alternating current (AC) or direct current (DC) signals. In either case, the regulator stabilizes the input signal and outputs the stabilized signal. The stabilized signal provides less noise and distortion for devices connected to the voltage regulator.
As complementary metal-oxide-semiconductor (CMOS) technology moves towards the use of deep submicron gate lengths, the amplitude of supply voltages used in analog, mixed signal, and radio frequency (RF) circuits is continuously decreasing. As the amplitude of the supply voltages decreases, the effects of noise on components fed by those supply voltages increase. For example, flicker noise (also known as 1/f noise) that occurs in semiconductor components will have a greater effect on low voltage supply signals.
FIG. 1 is a block diagram of a typical internally compensated low dropout regulator 10 for receiving an unregulated voltage input and outputting a regulated voltage signal. The example low dropout regulator 10 comprises an error amplifier 12 with associated output impedance Roe, a buffer amplifier 14, a regulation field effect transistor (FET) 16, a capacitor Cc, a capacitor Co, a resistor R1, a resistor R2, and a resistor Resr.
The error amplifier 12 of the illustrated example includes a negative input connected to a reference voltage Vref that includes its noise power spectral density (PSD) Sn,ref and a positive input connected to a feedback signal that includes noise PSD Sn,e. The error amplifier 12 is powered by an unregulated input voltage. The error amplifier 12 outputs an amplification of the difference between its positive input and the negative input. The output of the error amplifier 12 is connected to the resistor Roe which provides a voltage drop to a ground connection. The output of the error amplifier 12 and the resistor Roe are connected in parallel with a feedback signal provided by the capacitor Cc to the buffer amplifier 14. The buffer amplifier 14 isolates the impedance of its output from its input. The output of the buffer amplifier 14 includes the noise Sn,p. The regulation FET 16 is configured in common-source (CS) configuration. Due to its high transconductance value, input referred noise of the regulation FET 16 can usually be ignored.
The input referred noise power spectral density (PSD) of the error amplifier 12 and the buffer amplifier 14 is denoted by Sn,e(f) and the total output noise of the voltage reference is defined by Sn,ref(f). The total output noise spectral density of the low dropout regulator 10 is:
                                          S                          n              ,              o                                ⁡                      (            f            )                          =                                            (                                                                    S                                          n                      ,                      ref                                                        ⁡                                      (                    f                    )                                                  +                                                      S                                          n                      ,                      e                                                        ⁡                                      (                    f                    )                                                              )                        ⁢                                          (                                  1                  +                                                            R                      1                                                              R                      2                                                                      )                            2                                +                                                    S                                  n                  ,                                      R                    ⁢                                                                                  ⁢                    2                                                              ⁡                              (                                                      R                    1                                                        R                    2                                                  )                                      2                    +                      S                          n              ,                              R                ⁢                                                                  ⁢                1                                                                        [        1        ]            
Assuming that the reference noise Sn,ref(f) can be suppressed by sufficient filtering, the output noise of the low dropout regulator 10 including a filter can be approximated as:
                                          S                          n              ,              o                                ⁡                      (            f            )                          =                                                            S                                  n                  ,                  e                                            ⁡                              (                f                )                                      ⁢                                          (                                  1                  +                                                            R                      1                                                              R                      2                                                                      )                            2                                +                                                    S                                  n                  ,                                      R                    ⁢                                                                                  ⁢                    2                                                              ⁡                              (                f                )                                      ⁢                                          (                                                      R                    1                                                        R                    2                                                  )                            2                                +                                    S                              n                ,                                  R                  ⁢                                                                          ⁢                  1                                                      ⁡                          (              f              )                                                          [        2        ]            
As seen from equation [2], to minimize the output noise of the low dropout regulator 10, the noise contribution of the error amplifier 12 and the thermal noise contribution of the feedback network should be minimized. However, flicker noise of the error amplifier 12 becomes a dominant factor, especially for sub-micron processes at low frequencies.
The unity-gain frequency of the low dropout regulator 10 is limited by the parasitic pole generated by the output impedance of the error amplifier 12 and gate capacitance of the regulation FET 16. As shown in FIG. 1, this pole is split to a higher frequency by using the buffer amplifier 14 with low-output impedance between the error amplifier 12 and the regulation FET 16. This helps to reduce the output capacitance and improves settling time of the low dropout regulator 10.